MITIGATING TRAILING EDGE VOIDS IN FLEXOGRAPHIC PRINTING
A method for forming a flexographic plate for an image pattern including image features. The image pattern includes an array of image pixels, wherein the image pixels include printing pixels corresponding to portions of the image pattern where ink is to be printed on a substrate by the flexographic plate. Edge regions and interior regions of the image features are identified, which are separated by gap regions. A fine texture pattern is applied to the edge regions and a coarse texture pattern is applied to the interior regions to form a textured image pattern which is used to form the flexographic plate. No texture pattern is applied to the gap regions thereby leaving gaps between the edge regions and the interior regions of the image features.
The present invention relates to a method forming an image on a flexographic plate.
BACKGROUND OF THE INVENTIONIn graphic arts technology, a number of well-established printing processes utilize image carriers with a three-dimensional (3D) representation of data, the most popular of them being flexographic printing, which uses flexible relief plates or sleeves. A relief plate includes raised relief features, which are raised above the plate floor. It is the raised features that accept and transfer ink to the substrate. In a traditional flexographic prepress process with chemical etching, there is no possibility of fine control of relief properties other than the relief depth.
Flexographic printing uses a flexible relief plate 20 to print on a wide variety of substrates including paper, cardboard, plastic, and metal films. A simplified diagram of a flexographic printing press 30 is shown in
The process used to produce an image on a flexible relief plate 20 typically includes the following steps:
1. Expose the back of the plate to UV light;
2. Expose an intermediate film to the desired image;
3. Laminate the film to the top of the plate;
4. Expose the plate through the film using UV light;
5. Remove the film;
6. Use a solvent to wash away the unexposed plate material;
7. Apply additional exposure to harden the plate;
8. Dry the plate to remove as much of the solvent as possible.
The back exposure in step 1 is used to establish the floor 220 of the plate 20. The intensity of the exposure decreases as the illumination penetrates the plate because of absorption in the plate material. Once the intensity drops below a threshold value, there is insufficient cross linking in the polymer comprising the plate, and the remaining under-exposed polymer can be washed away. This is typically the top 0.5 mm of the plate. To form the relief, the front of the plate is exposed through an image layer with enough intensity so that sufficient cross-linking occurs all the way down to the plate floor 220.
For every opening in the image layer, a cone of UV light (typically with an angle of about 40 degrees from a normal to the plane) propagates through the plate 20 forming cone-shaped relief dots. A cross section of an exemplary plate 20 is shown in
Ink uniformity and density can be improved if a surface pattern or surface texture is applied to the flat tops of the relief pattern as shown in the
Such a fine texture pattern has an additional advantage in that it allows the edges of printing features to be well defined. The pattern does have its limits. When printing on plastic substrates, voids can appear in large features due to air entrapment. The pattern can also perform poorly if large volumes of ink need to be transferred to the substrate 22. To eliminate these problems, a coarser pattern is required. However, a coarser pattern will compromise edge definition.
In flexographic printing, large solid areas of a relief pattern can suffer from a number of artifacts. The ink can deposit unevenly, resulting in a reduction in ink density, and in the solid area having a mottled appearance. Ink can be squeezed off the relief feature near edges resulting in low ink density just inside the edge and high density just outside the edge. Air bubbles trapped between the plate 20 and substrate 22 can cause voids to appear at the trailing edge of large features.
Prior art exists to mitigate some these problems. For the case of voids due to air bubbles, U.S. Patent Application Publication 2010/0224091 to G. Zwadlo, entitled “Trailing edge pattern for relief plate feature,” describes a method that reduces voids by forming sunken patterns in the trailing edge of large print features.
U.S. Patent Application Publication 2016/0221379 to Y. Namba et al., entitled “Flexo printing plate,” attempts to deal with the voids by introducing small depressions in the large solid relief regions by having the density of the small depressions decrease with distance from the edge of the relief.
A preferred method combines a fine texture pattern at the edge of printing features with a coarser pattern in the interior of features as described in commonly-assigned U.S. Pat. No. 9,235,126 to R. Bielak et al., entitled “Flexographic surface patterns,” which is incorporated herein by reference This approach is illustrated in the textured image patterns 400 of
There remains a need for a method for introducing texture patterns to relief features in a flexographic printing plate which is simultaneously optimized for both uniform ink deposition and resistance to voids at the trailing edge of large relief features.
SUMMARY OF THE INVENTIONThe present invention represents a method for forming a flexographic plate including:
providing an image pattern including image features to be formed on the flexographic plate, the image pattern including an array of image pixels, wherein the image pixels include printing pixels corresponding to portions of the image pattern where ink is to be printed on a substrate by the flexographic plate;
identifying edge regions of the image features;
identifying interior regions of the image features;
wherein the edge regions and the interior regions are separated by gap regions;
providing a fine texture pattern;
providing a coarse texture pattern;
applying the fine texture pattern to the edge regions of the image features and applying the coarse texture pattern to the interior regions of the image features to form a textured image pattern, wherein no texture pattern is applied to the gap regions thereby leaving gaps between the edge regions and the interior regions of the image features; and
forming a flexographic plate using the textured image pattern.
This invention has the advantage that voids along the trailing edge of relief features are substantially eliminated by introducing a gap region between an edge region with a fine texture pattern and an interior region with a coarser texture pattern.
It has the additional advantage that the textured image pattern is simultaneously optimized for both uniform ink deposition and resistance to voids at the trailing edge of large relief features.
It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. Identical reference numerals have been used, where possible, to designate identical features that are common to the figures.
DETAILED DESCRIPTION OF THE INVENTIONThe invention is inclusive of combinations of the embodiments described herein. References to “a particular embodiment” and the like refer to features that are present in at least one embodiment of the invention. Separate references to “an embodiment” or “particular embodiments” or the like do not necessarily refer to the same embodiment or embodiments; however, such embodiments are not mutually exclusive, unless so indicated or as are readily apparent to one of skill in the art. The use of singular or plural in referring to the “method” or “methods” and the like is not limiting. It should be noted that, unless otherwise explicitly noted or required by context, the word “or” is used in this disclosure in a non-exclusive sense.
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure.
While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents.
When printing on certain plastic substrates such as low-density polyethylene (LDPE) using flexographic printing plates 20 voids can sometimes appear on the trailing edge of large solid relief areas. These voids are formed due to entrapment of air bubbles between the plate 20 and the substrate 22. It has been shown that this problem can be mitigated by providing a fine texture pattern 404 along the edges of the relief pattern 402 including slightly deeper valleys in the texture pattern, and providing a coarse texture pattern 408 including larger gaps between the peaks in the interior of the relief pattern 402 (see
It has been found that coarse texture pattern 408 in the interior regions of the relief features 402 of
A fine texture pattern 1216 is applied to edge regions specified by the edge pixel mask 1208 to create a fine-patterned edge structure 1224. In an exemplary embodiment, the edge pixel mask 1208 and the fine texture pattern 1216 are both binary images. In this case, the fine texture pattern 1216 can be applied by performing a logical “AND” operation to the corresponding pixels. In this case, a pixel in the fine-patterned edge structure 1224 will be “on” if the corresponding pixels in both the fine texture pattern 1216 and the edge pixel mask 1208 are both in the “on” state. Similarly, a coarse texture pattern 1220 is applied to interior regions specified by the interior pixel mask 1212 to create a coarse-patterned interior structure 1228.
In some cases, the image resolution that is used to perform the edge detection process 1206 may be different than the image resolution used to apply the texture patterns. For example, in an exemplary embodiment the image pattern 1204 may be a 2400 dpi pattern having pixels that are approximately 10×10 microns. The edge detection process 1206 can be applied at this resolution to produce an edge pixel mask 1208 and an interior pixel mask 1212 with this same resolution. The fine texture pattern 1216 and the coarse texture pattern 1220 can then be specified at some other resolution. For example, in an exemplary embodiment these texture patterns can be specified with pixels that are 5×10 microns such that two pixels are formed in the fine-patterned edge structure 1224 and the coarse-patterned interior structure 1228 for every pixel in the image pattern 1204. For example, this change in resolution can be accomplished by replicating each of the pixels in the edge pixel mask 1208 and the interior pixel mask 1212 in one direction before performing the AND operations.
The terms “fine” and “coarse” as used here are relative terms which reflect that the coarse texture pattern 1220 has a coarser texture than the fine texture pattern 1216. A texture can be said to be “coarser” if it has a lower dominant frequency, or if the average spacing between the “on” pixels (or groups of “on” pixels) is larger. For example, in the patterns of
The final step is to combine the fine-patterned edge structure 1224 and the coarse-patterned interior structure 1228 into a textured image pattern 1232. This can be accomplished by performing a logical “OR” operation to the corresponding pixels. In this case, a pixel in the textured image pattern 1232 will be “on” if a corresponding pixel in either the fine-patterned edge structure 1224 or the coarse-patterned interior structure 1228 is in the “on” state. The textured image pattern 1232 is used to form the flexographic plate 20 (
The edge detection process 1206 selects pixels in the image pattern 1204 to be part of the edge region or interior region based on their proximity to an edge of a relief feature. In an exemplary embodiment, the edge detection process 1206 uses a 3×3 pixel window 1320 and a 5×5 pixel window 1324 (with the corners removed) as illustrated in
The pixel windows 1320, 1324 are overlaid on the pixels of the image pattern 1204 (
In an exemplary embodiment, the textured image pattern 1232 is formed by replacing exposed pixels in the rendered image pattern 1204 (
While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents. The principles of the present invention may similarly be applied to other types of patterns or printing methods.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
PARTS LIST
- 10 ink
- 12 fountain roller
- 14 anilox roller
- 16 doctor blade
- 18 printing plate cylinder
- 20 plate
- 22 substrate
- 24 impression cylinder
- 26 fountain pan
- 30 flexographic printing press
- 204 raised feature
- 208 raised feature
- 212 raised feature
- 216 relief depth
- 220 floor
- 302 no surface texture pattern
- 303 conventional plate cell pattern
- 304 checkerboard surface texture
- 400 textured image pattern
- 402 relief feature
- 404 fine texture pattern
- 408 coarse texture pattern
- 600 plate forming system
- 604 digital front end (DFE)
- 608 imaging device
- 612 interface line
- 700 imaging system
- 704 cylinder
- 708 substrate
- 712 imaged data
- 716 advancement screw
- 720 imaging head
- 728 controller
- 732 imaging carriage
- 800 rendered image pattern
- 900 imaged substrate
- 1000 textured image pattern
- 1002 relief feature
- 1004 gap region
- 1008 fine texture pattern
- 1012 coarse texture pattern
- 1016 peak
- 1100 relief feature
- 1101 edge region
- 1102 gap region
- 1103 interior region
- 1104 A-A cross-section
- 1108 peak
- 1112 valley
- 1116 B-B cross-section
- 1120 peak
- 1124 valley
- 1128 valley
- 1204 image pattern
- 1206 edge detection process
- 1208 edge pixel mask
- 1212 interior pixel mask
- 1216 fine texture pattern
- 1220 coarse texture pattern
- 1224 fine-patterned edge structure
- 1228 coarse-patterned interior structure
- 1232 textured image pattern
- 1300 exposed pixel
- 1304 mask position
- 1308 mask position
- 1312 mask position
- 1316 mask position
- 1320 pixel window
- 1324 pixel window
Claims
1. A method for forming a flexographic plate comprising:
- providing an image pattern including image features to be formed on the flexographic plate, the image pattern including an array of image pixels, wherein the image pixels include printing pixels corresponding to portions of the image pattern where ink is to be printed on a substrate by the flexographic plate;
- identifying edge regions of the image features;
- identifying interior regions of the image features;
- wherein the edge regions and the interior regions are separated by gap regions;
- providing a fine texture pattern;
- providing a coarse texture pattern;
- applying the fine texture pattern to the edge regions of the image features and applying the coarse texture pattern to the interior regions of the image features to form a textured image pattern, wherein no texture pattern is applied to the gap regions thereby leaving gaps between the edge regions and the interior regions of the image features; and
- forming a flexographic plate using the textured image pattern
2. The method of claim 1, wherein the edge regions include image pixels along edges of the image features.
3. The method of claim 1, wherein the gap regions include image pixels adjacent to the edge regions.
4. The method of claim 1, wherein first and second pixel windows are used to determine whether an image pixel belongs to an edge region, an interior region or a gap region.
5. The method of claim 4, wherein the second pixel window is larger than first pixel window.
6. The method of claim 5, wherein a particular image pixel is identified as belonging to an edge region if the particular image pixel is a printing pixel and at least one of the image pixels in the first pixel window is not a printing pixel when the first pixel window is centered on the particular image pixel.
7. The method of claim 5, wherein a particular image pixel is identified as belonging to an interior region if the all image pixels in the second pixel window are printing pixels when the second pixel window is centered on the particular image pixel.
8. The method of claim 5, wherein a particular image pixel is identified as belonging to a gap region if the all of the image pixels in the first pixel window are printing pixels and at least one the image pixels in the second pixel window is not a printing pixel when the first and second pixel windows are centered on the particular image pixel.
9. The method of claim 5, wherein the first pixel window is a 3×3 pixel window and the second pixel window is a 5×5 pixel window.
10. The method of claim 9, wherein corner pixels of the 5×5 pixel window are removed.
11. The method of claim 1, wherein a width of the gap region is in the range of 5-30 microns.
12. The method of claim 1, wherein the image pattern is a binary image pattern.
13. The method of claim 12, wherein the image features include halftone dots, text characters or lines.
14. The method of claim 1, wherein the fine texture pattern is a checkerboard pattern and the coarse texture pattern is a sparse checkerboard pattern.
15. The method of claim 1, wherein the coarse texture pattern has a lower dominant frequency that the fine texture pattern.
Type: Application
Filed: Jun 7, 2017
Publication Date: Dec 13, 2018
Inventor: Richard Roman Bielak (Port Coquitlam)
Application Number: 15/616,134